A
return to model building and flying. Part 47

Note: if you
follow any
external links from this page, use the browser back button to return to
this page.

February 2008

Most of this was written before the tragic loss of my
father, So it may seem a pit sporadic at times, but I will get back
into some sort of routine eventually.

Everlasting tank

The
Buster engine tank setup continues to give me grief. Using an OS.25FX
and a Brodak 3oz metal tank (I wonder what that is in real money?....
88.7cc it turns out.), it was giving very unpredictable runs with
exhaust pressure or not.

Dropping the tank by about 10mm eased
the cutting out inverted problem, but the engine is very vague to try
and set. The run consists of a very rich takeoff and a very lean and
much to fast for comfort middle and end of run. All of this with a
seeming magical ability of the engine to run on empty tank full of
fumes.Believe it or not powered landings are more common than
deadstick. This resulted in landing the model at the end of one flight
and wondering why after it stopped it was balanced on the wheels with
the tail in a flying attitude. It was only when it nosed forward and
the sods of earth flew into the air that I realised, the engine was
still running.....!

It's was time for
..................................!

The
cunning plan, "No it doesn't involve a potato Baldric," was to utilise the existing RC carb to restrict the top
end performance, then if the engine leaned out there is a limit to how
fast it could rev.

This was a partial
success in that I ended up with the throttle half closed and it would
still get through he schedule but the great difference in mixture
between tank full and empty still exists. I am getting a little tired
of constantly having to modify existing tanks to uniflow to get round
this problem, and to be honest if doesn't always work with peaky
engines; that covers most Schnuerle ported engines which only seem to be happy at high revs.

I'm coming to the conclusion that to
use a smaller engine and just let it rip, using just the choke to limit
the top end, may be a better approach; it's certainly simpler.

This
is how I did it. The strange bends in the pushrod are due to the O.S.
practice of making actuating levers that are so close to the carb body
that a strait pushrod fouls both the mounting bolts and the port
casting in the crankcase. The final kink is to keep the electric plug
leg in a place where I can easily adjust it on the field.

The screw lock is made from a 13 amp mains plug leg, with one
end drilled to take la long backplate bolt. You may have to use
something else if you happen to live in Europe or the US, and wanted to
do the same thing.

The
carb aperture can be set to a known reference diameter by using a
drill shank; in this case 4.5 mm. Useful when it comes to making
a fixed aperture venturi, as it gives some idea of where to start.

The tank run at half
throttle is endless on an 88cc tank, I have replaced it with a smaller
one, only to find it too small. Ever find that something just doesn't
want to play....!

Flying

Not much, due to the weather, almost constant rain and high
wind for most of January, plus I had other more pressing things to worry
about.

SIG Manufacturing

2nd of January and money was extracted
from my bank account. 14th props arrived. Now I know it can be done, whether it's worth the hassle is another matter.

Hellcat

After playing with the
MDS 48 for a few flights. the throttle response seems to have quite a
lag that I can't quite get right. After a lot of good humoured badgering from the RC
fixed wing and heli fliers to try using 30% Nitro; I will add a this point that as the engine
has a quite high compression rating I didn't expect it to have much
effect, I capitulated; I must confess I was curious to see the results
myself.

The result was an engine that I could not really set the main
needle, it was so woolly it could be set all over the place. Set
lean, it would still run but at reduced revs with an increase in the
mechanical clatter, obviously
pre-ignition. in the air it had a similar performance to running on 5% Nitro at half
throttle, whilst consuming fuel at a prodigious rate.

A useful experiment, even if only proved that my assumptions were correct.

I have come to the conclusion that the there is not much
advantage over the SC 40 I was using before, so The old SC engine is
now back
in the Hellcat. I just wish I could find out why the MkIII SC 40 has
such a weather dependant throttle response, as it's certainly the most
powerful 40 I have, but for competition use, this behaviour rules it
out.

F8F Bearcat

No Progress on the model

Ultimate

No Progress on the model

Next month:
(Hopefully)

A little peek at "Perky Speed", an
intriguing concept from the USA. And if the wind EVER! stops blowing
during February; some flying. Don't think I have ever known the wind
blow so strong for so long before.